1. Field of the Invention
The present invention relates to piezoelectric motors. More particularly, the present invention relates to a unique configuration for mounting piezoelectric elements in a piezoelectric wave motor.
2. Description of Related Art
Piezoelectric motors are utilized in a wide variety of applications in modern society, such as autofocusing camera lenses and automatic control units for window blinds. Piezoelectric motors are particularly well-suited for any application which requires a motor having a compact size (e.g. as small as the size of a fingertip), quiet operation, high torques at low speeds, quick response, and/or which is not effected by magnetic fields.
A typical design of an existing piezoelectric motor is shown in FIG. 1 designated by reference numeral 10. Motor 10 is a rotary motor that includes a disk-shaped stator 12 having a comb-tooth top surface 14 and a flat bottom surface 16. Motor 10 also includes a thin piezoelectric ring 18, which is bonded to bottom surface 16 of stator 12 with an adhesive material, such as an epoxy resin. Dispersed around ring 18 are individual segments of piezoelectric ceramic which have been electrically poled in alternating opposite directions (indicated by "+" and "-") along an axis of poling which is perpendicular to the plane of stator 12.
Piezoelectric motor 10 further includes a disk-shaped rotor 20, here shown as a geared rotor, which, together with stator 12 and piezoelectric ring 18 bonded to the stator, are mounted onto a shaft 22 which extends upwardly from the center of a rigid base 24. A spring washer 26, bearing 28, and e-clip 30 function to hold rotor 20 in pressure contact with top surface 14 of stator 12. A thin friction liner 32 is placed between stator 12 and rotor 20 to reduce sliding, energy losses during the operation of motor 10. (It is known in the art to attach friction liners to rotors and further references to rotors herein will be understood to include a possible friction liner.)
A high frequency a.c. voltage drive source 34 is also provided to drive motor 10. A first electrical lead 36 supplies a first a.c. voltage signal (typically, V.sub.o sin .omega.t) to a first set of poled segments on ring 18, and a second electrical lead 38 supplies a second a.c. voltage signal (V.sub.o cos .omega.t) to a second set of poled segments on ring 18, which are displaced along the stator from the first set as is known in the art. A third electrical lead 40 is connected to ground.
In operation, the a.c. voltage signals from drive source 34 cause the poled segments of piezoelectric material in ring 18 to expand and contract in such a manner that a traveling wave is generated in stator 12. The comb-tooth top surface 14 of stator 12 amplifies this traveling wave, and the crests of the amplified traveling wave move in an elliptical motion such that a tangential force is created at the wave crests. As the wave crests contact rotor 20, this tangential force causes movement of rotor 20 to thereby drive motor 10.
Motor 10 has all of the desirable features which are generally associated with piezoelectric motors (e.g. compact size, quiet operation, high torques at low speeds, quick response, and not effected by magnetic fields); however, there are still several shortcomings associated with the design and operation of motor 10.
First, the expansions and contractions of the individual segments (i.e., individual piezoelectric ceramics--typically referred to as elements) of ring 18 create alternating tensile and compressive stresses in the elements. Because piezoelectric elements are ceramics, and are typically weak in tension, these alternating tension stresses promote the growth of cracks within the elements. Over time, these cracks will decrease motor reliability and may eventually lead to the failure of motor 10.
Second, motor 10 is driven by the expansions and contractions of the poled segments of piezoelectric element ring 18, which expansions and contractions are transverse to the element's axis of poling. (Expansions and contraction transverse to the piezoelectric element's axis of poling are commonly referred to as being in the "d.sub.31 direction"). It is well known in the art that expansions and contractions parallel to the element's axis of poling (commonly referred to as the "d.sub.33 direction") are approximately twice the magnitude of those in the d.sub.31 direction for a given electrical field. Thus, motor 10 does not fully utilize the piezoelectric properties of the elements.
Third, in order to transmit forces to the stator 12, piezoelectric element ring 18 is directly bonded to stator 12 such that shear stress is placed on the bond as the segments of ring 18 expand and contract in such a manner that a traveling wave is generated in stator 12. Over time, this shear stress on the bond between ring 18 and stator 12 may lead to the failure of motor 10.